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I understand that is you extend the bellows beyond a certain point you will need to calculate adjustments for exposure compensation.
Is there a fast rule (to simplify using in the field) as to the compensation formula.
I have seen a quickdisk, which would work for close-ups. Is this really a significant problem for landscape work? (inifinity)
Thanks
This is not a significant problem for landscape photography. It only starts becoming noticable when you start making images that are .25X lifesize and progressively higher magnifications.
In theory, any bellows extension requires compensation to exposure, but as the previous poster says, this doesn't matter unless you are doing close-ups. To calculate the exposure, take the focal length of the lens and square it (e.g. 210(mm)x210=44100. Now take the total extension (distance from rear of lens to film) and square that (e.g. 400mm from lens to film, 400x400=160000. Now divide focal length into extension (160000/44100)=3.628. Your exposure factor is therefore 3.628, or a bit over 1 1/2 stops. If it's any help I have a D.O.F. chart that calculates this automatically, can email it if required.
If you work in F stops, you don't have to square anything.Divide the focal length of the lens into the total bellows extension, take away one, and that's the fraction of a stop, or number of stops, that you need to open the lens or extend the shutter speed by.For example: 150mm lens with 240 mm bellows draw. 240 divided by 150 is 1.6, minus 1 leaves 0.6. Therefore; open up 0.6 stops (2/3rds stop is close enough).
There's an even easier way.
If an object appears life size on the ground glass, then we know the increse in exposure is 4x or 2 stops, or PDC. To get that object at 1:1 you need 2x the focal length bellows extension (or again, PDC).
So a 10" lens at infinity has 10" of extension and therefore no exposure compensation. At 1:1 it has 20" of extension and requires 2 stops or 4X exposure compensation.
So, at 15 inches extension, it would require 1 stop increase. At 12.5 it would require 1/2 stop increase.
So, for a given lens, add 1/2 stop for every 1/4 (25%) increase in extension. Works with inches or millimeters. I carry a tiny little 36" tape measure I got as a giveaway at a hardware store for this reason in my camera case.
However as Ellis points out, Kodak and others have published the general rule that if the camera is focused NO closer than 8x (or 9x or 10x according to some sources) the lens focal length, then the increase in exposure is insignificant.
In any even Calumet and others have a nice little device that consists of a square 1" piece of plastic and a ruler that applies the above approach.
You don't have to do all that calculating. If the bellows is extended 1 focal length (focussed on infinity) no change in exposure is required. If the bellows is extended 1.4 focal lengths (1.5 is probably accurate enough) increase exposure by 1 stop. If the bellows is extended 2 focal lengths, increase exposure by 2 stops. Everything in between can be interpolated.
Or make it really easy on yourself and us the f stop method. If you are shooting with an 8 inch lens and your bellows increases to 11 inches, that would be the next f stop, so add a stop, from there if it increased to 16 inches add another stop. If it increases to 22 inches add another stop. If you start with an 11 inch lens and the bellows incrased to 16 inches, add a stop, and so on and so forth. For in between, interrpolate, i.e. 9 inches would be i/3 or a stop increase for an 8 inch lens becasue it is one third of the way to f 11. One thing though that nobody else mentioned. That is that you have to watch how you are adding exposure. If you always do it with f stops you of course lose depth of field, whereas if you do it with exposure time you will usually be okay except that when your exposures start to get long and you start doubling them you might now start to have to also worry about reciprocity. So for example, for a given film it may be preferable to add a stop of exposure by opening the diaphram by a stop even though you are losing some depth of field rather than to go from 1/2 second to 1 second because at 1 second that film might call for another second of exposure due to reciprocity characteristics and you may not want a 2 second exposure due to wind or other movement. Good luck. kevin
Or go here, download it, print it, stick it on carboard, follow the simple instructions, keep it in your camera bag and never worry about bellows compensation again! Works like a charm - make half a dozen so you won't lose em...
http://www.salzgeber.at/disc/index.html
Tim
Tim's right about the disc. I used it to shoot some close-ups of fallen leaves in Zion last November. The disc said to open up exactly one f-stop, my conscience told me that wasn't enough. Guess which one was right. The disc exposure was absolutely perfect!
Or don't worry about it much at all with landscapes. A rule of thumb is that you don't need any compensation unless the nearest object is closer than 10 times the focal length of the lens. I've also seen the rule of thumb as 8 times the focal length of the lens. With a 210 mm lens, and using the 10 times "rule," the closest object would have to be 84 nches (7 feet) or less before you became concerned. That's pretty close and I don't personally encounter that situation very often with landscape work though others might differ.
I use a very handy plastic laminated FotoSharp Guide, "Macro and Closeup Exposure Adjustments". It covers focal lengths from 75mm to 300, subject magnification from 1/8th to 1:1 to 3:1, distance(s) from subject in inches, and gives adjustments in Fstops or time changes. Fits in my back pocket. These are available on Ebay all the time. Or direct from Bob at 800 361-8341. I don't know Bob, and have no interest in his enterprise...hope it is ok to post this
I do a lot of close work, and this has never failed me, yet. It saves me one more right brain to left brain shift. I would much rather concentrate on the delicate drawing of light on a petal, than start doing squares and dividing large numbers (or searching for a calculator battery). Meanwhile, the subject has wilted, the wind has come up, the bulb has blown, the light has changed...